The production of oil and natural gas depends on natural pressure within the earth formation at the bottom of a well bore, as well as the mechanical efficiency of the equipment and its configuration within the well bore to move the hydrocarbons from the earth formation to the surface. The natural formation pressure forces the oil and gas into the well bore. In the early stages of a producing well when there is considerable formation pressure, the formation pressure may force the oil and gas entirely to the earth surface without assistance. In later stages of a well's life after the formation pressure has diminished, the formation pressure is effective only to move liquid and gas from the earth formation into the well. The formation pressure pushes liquid and gas into the well until a hydrostatic head created by a column of accumulated liquid counterbalances the natural earth formation pressure. Then, a pressure equilibrium condition exists and no more oil or gas or water flows from the earth formation into the well. The hydrostatic head pressure from the accumulated liquid column chokes off the further flow of liquid into the well bore, causing the well to “choke off” or “die,” unless the accumulated liquid is pumped or lifted out of the well.
By continually removing the liquid, the hydrostatic head pressure from the accumulated column of liquid remains less than the natural earth formation pressure. Under such circumstances, the natural earth formation pressure continues to move the liquid and gas into the well, allowing the liquid and gas to be recovered or produced. At some point when the natural earth formation pressure has diminished significantly, the cost of removing the liquid diminishes the value of the recovered oil and gas to the point where it becomes uneconomic to continue to work the well. Under those circumstances, the well is abandoned because it is no longer economically productive. A deeper well will require more energy to pump the liquid from the well bottom, because more energy is required to lift the liquid the greater distance to the earth surface. Deeper wells are therefore abandoned with higher remaining formation pressure than shallower wells.
To keep a well in production, it is necessary to remove the accumulated liquid to prevent the liquid from choking off the flow of gas. Because a considerably greater volume of gas is usually produced into a well compared to the amount of liquid produced into the well, the greater volume of gas can be recovered more economically by removing a relatively lesser volume of the accumulated liquid. Consequently, there may be an economic advantage to recovering natural gas at the end of a well's lifetime, because the gas is more economically recovered as a result of removing a relatively smaller amount of accumulated liquid. These factors are particularly applicable to recovering gas from relatively deep wells.
Gas pressure lift systems have been developed to lift liquid from wells under circumstances where mechanical pumps would not be effective or not sufficiently economical. In general, gas pressure lift systems inject pressurized gas into the well to force the liquid up from the well bottom, rather than rely on mechanical pumping devices to lift the liquid. The injected gas may froth the liquid by mixing the heavier density liquid with the lighter density gas to reduce the overall density of the lifted material. Alternatively, “slugs” or shortened column lengths of liquid are separated by bubble-like spaces of pressurized gas, again reducing the overall density of the lifted material. In both cases, the amount of energy required to lift the material is reduced, or for a given amount of energy it is possible to lift material from a greater depth.
One problem with injecting pressurized gas into a well casing is that the pressurized gas tends to oppose the natural formation pressure. The injected gas pressure counterbalances the formation pressure to inhibit or diminish the flow of liquids and natural gas into the well. Once the injected gas pressure is relieved, the natural earth formation will again become effective to move the liquid and gas into the well. However, because the casing annulus is pressurized for a significant amount of time during each production cycle, the net effect is that the injected gas pressure diminishes the production of the well. Stated alternatively, producing a given amount of liquid and gas from the well requires a longer time period to accomplish. Such reductions in the production efficiency in the later stages of the well's life may be so significant that it becomes uneconomical to work the well, even though some amount of hydrocarbons remain in the formation.
One type of pressurized gas lift apparatus, method and gas recovery cycle which is particularly advantageous for use with wells having relatively low down-hole natural earth formation pressure is described in the above-identified U.S. patent. In that technique, a three chamber evacuation phase is included in each gas recovery cycle to create a relatively low pressure throughout the well and thereby augment the natural earth formation pressure to draw more gas and liquid from the surrounding earth formation into the bottom of the well. The relatively low pressure is communicated from the earth surface down into the well through a casing chamber, a production chamber and a lift chamber. Liquid is forced from the casing chamber into the production and lift chambers and is then lifted to the earth surface through the lift chamber by applying a relatively high pressure to the production chamber. A one-way valve at the bottom of the production chamber allows fluid to flow from the casing chamber into the production chamber, but the one-way valve confines the relatively high pressure in the production chamber when the liquid is lifted up the lift chamber to the earth surface. After the liquid is lifted in this manner, at least a significant portion of the gas is produced through the same path up the casing chamber, down the production chamber and then up the lift chamber.
The three chamber evacuation phase in the gas recovery cycle is particularly advantageous in improving the efficiency and maintaining the productivity of relatively deep wells having relatively low natural earth formation pressures and which produce liquid at a relatively low rate. Because liquid is produced at a relatively low rate, it is possible to use the three chamber evacuation phase as a primary gas production phase. The gas is produced directly up the casing chamber, and the gas is not subject to the flowing friction losses created by the relatively lengthy flow path down the smaller diameter production chamber and then up the even smaller diameter lift chamber. The flowing friction losses through the shortest flow path and largest diameter casing chamber are substantially less than the more circuitous and friction-engendering path up the casing chamber, down the production chamber and then up the lift chamber.
The technique of the above-identified U.S. patent is best implemented in these low earth formation pressure-low liquid production wells by minimizing the amount of time or proportion of each gas recovery cycle required to perform the liquid capture, liquid removal and production phases during which the liquid is removed from the casing chamber and lifted to the earth surface. The relatively low rate of liquid production by the well permits minimizing these phases while maximizing the more efficient gas producing three chamber evacuation phase.